Evidence has been accumulated that changes in the intracellular concentration of calcium is important in the regulation of a number of hormone-sensitive metabolic processes. Such hormonal effects on cellular metabolism could be brought about by modulating the normally low intracellular calcium concentration by altering the activity of the calcium-pumps that are located at the plasma membranes, mitochondria and endoplasmic reticulum. The principal investigator has identified and characterized the calcum-pump in plasma membranes of rat liver and has recently shown a 2-3 fold increase in its activity following the chemical induction of diabetes. Administration of exogenous insulin to the diabetic rats restored the activity of the calcium-pump to control values. In addition, the effect of diabetes on this Ca2+ pump could be mimiced in a dose dependent manner in vitro by the catalytic subunit of the cAMP-dependent protein kinase. This proposal aims at extending and characterizing these preliminary observations and investigating its mechanism. The proposed studies are organized into 3 parts: 1. characterization of the in vivo effect of diabetes on plasma membrane calcium transport activity in liver. 2. effect of in vitro hormone replacement on calcium transport activity in plasma membranes from hepatocytes. 3. mechanism by which diabetes and hormone replacement regulate the calcium transport activity in plasma membranes of rat liver. The ultimate information gained from this proposed study could contribute significantly towards understanding the role of the plasma membrane calcium-pump in the aberrant hepatic metabolism of diabetic animals, and in the mechanism of insulin action.